Valve Projects

12AX7 Preamp

The 12AX7 is a popular dual triode vacuum tube and it ended up being the basis for my first valve project, which you can read about in this section. The preamp is based on a project that can be found at the following site:

http://www.shine7.com/audio/12ax7_pre.htm

Please do check out the site for additional information on how the preamp works.

The preamp is split into three main parts:

1. Low voltage supply for the valve heaters

2. High voltage supply for the valve anodes

3. The preamp

It is a good idea to build and test the low voltage supply first, then the high voltage supply, then lastly the actual preamp.

Consult the circuit diagram below:

Note that valve numbering is from the bottom of the tube.

One way to obtain the high voltage supply is to take the mains input supply and rectify it but I feel that is not safe as you do not get the isolation of a transformer. The better approach is to use an AC low voltage power supply and rectify, smooth and regulate to 6V for the valve heaters. The 12AX7 has two heaters which can be powered in series or parallel but in this circuit they are wired in parallel so they can be run off 6V; they draw about 300mA.

To get the high voltage supply the 9V AC input is stepped up using a transformer which would normally be used to lower the mains input but in this circuit the 9V is supplied to the transformer primary and the secondary will give out about 200V. This high voltage from the transformer secondary is rectified and smoothed which includes an RC filter. Be very careful around the high voltage capacitors as they can deliver a nasty shock long after the power supply has been turned off. It is a good idea to discharge the capacitors after power has been turned off by placing a low value, high power resistor in parallel with C6. Alternatively, you can solder a high value, medium power resistor in parallel with C6 so it drains the capacitors when power is turned off.

The preamp is single-ended because it only uses one amplifying component, which is one of the two triodes in the 12AX7. The second half (the right-hand side) is a cathode follower which changes the output impedance of the first stage from about 51KR to about 625R at the output. The volume control (VR1) is at the input as that is what seems to be the preferred by designers but there is no reason it cannot be at the output or even between circuit stages.

To test, first of all make sure the low voltage and high voltage power supply circuits are working; measure with a suitably rated multimeter or oscilloscope. The 12AX7 hardly glows so that isn't a good indicator that it is getting power so instead check with a multimeter set to current measurement; the two heaters should draw around 300mA. The actual preamp part of the circuit draws just 1.4mA which may not seem much but you have to take into account the high voltage supply, which increases the total power used.

If you want to further test the amp by listening to audio there is no reason why you can't feed an audio source such as an MP3 player to the input. However, to drive a loud speaker at the output you will need an audio transformer to change the impedance to that of the loudspeaker's. It is possible to use a power transformer as an audio transformer but it generally is not a good idea as they are not designed for audio. Instead of using a transformer and loudspeaker you can connect the preamp output to a power amp with the preamp volume control set not too high (otherwise you may get distortion).

You can also do tests on the preamp by using a function generator and oscilloscope as by feeding a 1V p-p sine or square wave of varying frequencies and observing the output waveform compared to the input waveform. The output should be about 50V p-p, of similar frequency and 180 degrees out of phase of the input. Because a square wave can be thought of as consisting of multiple sine waves at different frequencies the square wave effectively lets us test an amp with multiple sine waves simultaneously.

You can view a video I made about the preamp at:

https://youtu.be/IR3pPCEXU6k

All content of this and related pages is copyright (c) James S. 2016